High-strength nodular iron with good weldability and machinability

ABSTRACT

A nodular iron alloy and automotive components, such as differential and drive axle components, are provided. The nodular iron alloy may include iron, about 3.1-3.3 wt % carbon, about 2.7-4.3 wt % silicon, about 0.15-0.40 wt % manganese, about 0-0.10 wt % magnesium, about 0-0.2 wt % nickel, about 0-0.4 wt % copper, about 0-0.30 wt % chromium, about 0-0.03 wt % phosphorus, and about 0-0.02 wt % sulfur. The nodular iron alloy may have an ultimate tensile strength of at least 620 MPa as-cast. This alloy possesses favorable weldability to weld with steel components without substantial preheating or post heat treatment for a strong and tough weldment, and it has good machinability to facility comprehensive machining operations.

FIELD

The present disclosure relates generally to iron alloys, and more particularly, to iron alloys that are nodular and have a desired strength for weldability and machinability, as well as components made therefrom, such as differential and drive axle components.

INTRODUCTION

Welded front wheel drive differential carriers reduce both weight and cost compared to conventional bolt-joined structures. However, traditional high-strength ductile irons, such as SAE D5506 and D7003 have poor weldability to steel and poor machinability due to a high percentage of pearlite in the matrix (e.g., about 70-100%). A high concentration of pearlite generally results in low weldability, low machinability, and low fracture toughness. On the other hand, for ductile irons, the more of the pearlite phase, the higher the strength. High strength is generally desired in many automotive components.

Low toughness of the weld heat affected zone (HAZ) in ductile iron makes the weld very sensitive to welding cracking. For example, because of a high pearlite volume in D7003, welding is very difficult without complex welding procedures, such as those involving preheating and post-welding heat treatment.

SUMMARY

This disclosure provides a new high-strength nodular iron alloy that has desirable weldability and machinability. The new nodular iron alloy may have, for example, an ultimate tensile strength of at least 620 MPa and an elongation of at least 6%. The high-strength nodular ductile iron alloy may have a ferrite/pearlite matrix with the pearlite phase being no greater than 35%. Weld heat affected zone (HAZ) toughness and fatigue strength are enhanced, and machining costs may be lowered though the use of this new ductile iron alloy.

In one example, which may be combined with or separate from the other examples and features provided herein, a nodular iron alloy is provided containing: iron, about 3.1 to about 3.3 weight percent carbon, about 2.7 to about 4.3 weight percent silicon, and about 0.15 to about 0.40 weight percent manganese.

In another example, which may be combined with or separate from the other examples given herein, a nodular iron alloy is provided that consists essentially of: about 3.1 to about 3.3 weight percent carbon, about 2.7 to about 4.3 weight percent silicon, about 0.15 to about 0.40 weight percent manganese, 0 to about 0.10 weight percent magnesium, 0 to about 0.2 weight percent nickel, 0 to about 0.4 weight percent copper, 0 to about 0.30 weight percent chromium, 0 to about 0.03 weight percent phosphorus, 0 to about 0.02 weight percent sulfur, and the balance iron.

Additional features may be optionally provided, including but not limited to the following: wherein the silicon is provided in an amount of about 3.5 to about 4.1 weight percent; wherein the iron is provided in an amount of at least 86.55 weight percent; the nodular iron alloy further comprising magnesium in an amount not exceeding 0.10 weight percent; the nodular iron alloy further comprising nickel in an amount not exceeding 0.2 weight percent; the nodular iron alloy further comprising copper in an amount not exceeding 0.4 weight percent; the nodular iron alloy further comprising chromium in an amount not exceeding 0.30 weight percent; the nodular iron alloy further comprising sulfur in an amount not exceeding 0.02 weight percent; the nodular iron alloy further comprising phosphorus in an amount not exceeding 0.03 weight percent; wherein the nodular iron alloy has an ultimate tensile strength greater than 620 MPa as-cast; the nodular iron alloy having an elongation of greater than 6%; wherein the nodular iron alloy is substantially free of cobalt and molybdenum; wherein the iron is present in an amount of 65-85% of a ferrite microstructure and in an amount of 15-35% of a pearlite microstructure; and wherein the iron surrounds a plurality of graphite nodules.

Further additional features may be included, including but not limited to the following: an automotive component being created from any variation of the nodular iron alloy; and the automotive component being a differential component or a drive axle component.

The above features and advantages, and other features and advantages of the present disclosure, will be readily apparent from the following detailed description of the many aspects of the present disclosure when taken in connection with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are provided for illustration purposes only and are not intended to limit this disclosure or the claims appended hereto.

FIG. 1 is an enlarged view of a nodular iron alloy illustrating the microstructure thereof, in accordance with the principles of the present disclosure; and

FIG. 2 is a perspective view of a differential assembly having components formed of a nodular iron alloy in accordance with the principles of the present disclosure.

DETAILED DESCRIPTION

Nodular ductile iron alloys having desirable strength, weldability, and machinability are provided. These nodular iron alloys are particularly useful for cast automotive components that undergo large loads, fatigue, and a significant amount of machining operations, as well as being welded to another component. The automotive components may be implemented as cast, which saves on additional steps and costs. For example, components formed of the disclosed nodular iron alloy may be laser-welded to steel without preheating or post-welding heat treatments. Heat affected zone hardness/brittleness may be lower than that of conventional iron alloys, resulting in an improvement to the welded zone's fracture toughness and fatigue properties.

The nodular iron alloys disclosed herein contain iron, carbon, silicon, manganese, and the nodular iron alloys may also contain phosphorus, sulfur, nickel, copper, chromium, and magnesium.

The nodular iron alloys disclosed herein may include iron and by weight about 3.1 to about 3.3 weight percent (or exactly 3.1-3.3 wt %) carbon, by weight about 3.5 to about 4.1 weight percent (or exactly 3.5-4.1 wt %) silicon, by weight about 0.15 to about 0.40 weight percent (or exactly 0.15-0.40 wt %) manganese. In some cases, the silicon may be provided in amounts as low as about 2.7 weight percent or as high as about 4.3 weight percent, and a carbon equivalent of about 4.2 to about 4.4 weight percent is maintained.

The iron may be provided in an amount of at least 86.55 weight percent. The nodular iron alloys may also include one or more of the following: magnesium in an amount not exceeding 0.10 weight percent; nickel in an amount not exceeding 0.2 weight percent; copper in an amount not exceeding 0.4 weight percent; chromium in an amount not exceeding 0.30 weight percent; phosphorus in an amount not exceeding 0.03 weight percent; and sulfur in an amount not exceeding 0.02 weight percent. For example, Table 1 shows a first example of the nodular iron alloy, which contains iron, carbon, silicon, manganese, and which may also contain phosphorus, sulfur, magnesium, nickel, copper, and chromium. The iron may be provided in an amount of at least 86.55 weight percent.

TABLE 1 First Example of a New Nodular Iron Alloy Carbon C Si Mn P S Cu Ni Cr Mg Equivalent (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) CE (wt %) Fe 3.1-3.3 2.7-4.3 0.15-0.40 0-0.03 0-0.02 0-0.4 0-0.2 0-0.30 0-0.10 4.2-4.4 Balance

In another example, Table 2 shows an example of the nodular iron alloy, which contains iron, carbon, silicon, manganese, and which may also contain phosphorus, sulfur, magnesium, nickel, copper, and chromium. In this second example, the silicon has a smaller range of possible amounts, by weight percent. As before, the iron may be provided in an amount of at least 86.55 weight percent.

TABLE 2 Second Example of a New Nodular Iron Alloy Carbon C Si Mn P S Cu Ni Cr Mg Equivalent (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) CE (wt %) Fe 3.1-3.3 3.5-4.1 0.15-0.40 0-0.03 0-0.02 0-0.4 0-0.2 0-0.30 0-0.10 4.2-4.4 Balance As with Table 1, it should be understood that the new nodular iron alloy can have any combination of the listed elements above in Table 2, and need not include all of them.

Referring now to FIG. 1, a nodular iron alloy 10 is illustrated, which may have a microstructure developed from a composition such as that of Table 1. A plurality of graphite nodules 12 are present, and iron 14 surrounds the graphite nodules 12. Of the iron 14 surrounding the graphite nodules 12, 65-85% of the iron has a ferrite microstructure 16, and only 15-35% has a pearlite microstructure 18. In one example, the iron 14 may be provided with about 20% being pearlite and with about 80% being ferrite. Generally, the pearlite microstructure 18 surrounds the ferrite microstructure 16, and the ferrite microstructure 16 is disposed directly adjacent to the nodules 12. The ferrite microstructure 16 appears lighter than the pearlite microstructure 18 in FIG. 1, upon a regular etching procedure for metallographic characterization.

The nodular iron alloy 10 may have an ultimate tensile strength, for example, of at least 620 MPa, as-cast. In some examples, the ultimate tensile strength of the as-cast nodular iron alloy may be in the range of about 620 to about 700 MPa. Accordingly, the nodular iron alloy 10 has sufficient strength for use in high-load automotive components, such as differential and drive axle components, but is also machinable and weldable. The nodular iron alloy 10 may have, for example, at least 6% elongation.

The nodular iron alloys described herein may be used to manufacture an automotive component, which may be, in some cases, a cast automotive propulsion system component. Therefore, it is within the contemplation of the inventors herein that the disclosure extends to automotive components, including but not limited to differential components, drive axle components for both front and rear axles, axle shafts, and the like. For example, referring to FIG. 2, a differential assembly 200 is illustrated, which may have components made of any variation of the nodular iron alloy described herein, and which may be cast. For example, the differential assembly 200 has a differential carrier 202 that may be formed of a variation of the nodular iron alloy described herein. As such, the differential carrier 202 may be effectively welded via weld joints 204, 206 to adjacent components. More specifically, the differential carrier 202 may be welded via an axial weld joint 204 to an external ring gear 208 for further connection to a transmission assembly. In the alternative, the components could be arranged such that the weld joint 204 could be a radial weld joint. Further, the differential carrier 202 may be welded, for example, by a radial weld joint 206 to another component, such as the cover 210. In this example, the external ring gear 208 and the cover 210 are formed of steel, and the differential carrier 202 formed of the nodular iron alloy disclosed herein forms a good weld with these steel components.

The differential carrier 202 may house side gears 212, 214 that are connected to half shafts 216, 218 and differential pinion gears 220, 222 that are connected to that may be connected to the shaft 224. Any of the components of the differential assembly 200 may be formed of any variation of the nodular iron alloy disclosed herein. Accordingly, the components of the differential assembly 200, such as the differential carrier 202, are machinable, weldable, and of sufficient strength to be used in automotive applications. The present iron alloys may also be useful in gas and oil industry components and general industrial components.

Furthermore, while the above examples are described individually, it will be understood by one of skill in the art having the benefit of this disclosure that amounts of elements described herein may be mixed and matched from the various examples within the scope of the appended claims. It is further understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. 

What is claimed is:
 1. A nodular iron alloy comprising: iron; about 3.1 to about 3.3 weight percent carbon; about 2.7 to about 4.3 weight percent silicon; and about 0.15 to about 0.40 weight percent manganese.
 2. The nodular iron alloy of claim 1, wherein the silicon is provided in an amount of about 3.5 to about 4.1 weight percent.
 3. The nodular iron alloy of claim 2, wherein the iron is provided in an amount of at least 86.55 weight percent.
 4. The nodular iron alloy of claim 3, further comprising magnesium in an amount not exceeding 0.10 weight percent.
 5. The nodular iron alloy of claim 4, further comprising nickel in an amount not exceeding 0.2 weight percent.
 6. The nodular iron alloy of claim 5, further comprising copper in an amount not exceeding 0.4 weight percent.
 7. The nodular iron alloy of claim 6, further comprising chromium in an amount not exceeding 0.30 weight percent.
 8. The nodular iron alloy of claim 7, further comprising sulfur in an amount not exceeding 0.02 weight percent.
 9. The nodular iron alloy of claim 8, further comprising phosphorus in an amount not exceeding 0.03 weight percent.
 10. The nodular iron alloy of claim 3, wherein the nodular iron alloy has an ultimate tensile strength greater than 620 MPa as-cast.
 11. The nodular iron alloy of claim 10, having an elongation of greater than 6%.
 12. The nodular iron alloy of claim 3, wherein the nodular iron alloy is substantially free of cobalt and molybdenum.
 13. The nodular iron alloy of claim 11, wherein the iron is present in an amount of 65-85% of a ferrite microstructure and in an amount of 15-35% of a pearlite microstructure, wherein the iron surrounds a plurality of graphite nodules.
 14. An automotive component created from a nodular alloy according to claim
 3. 15. The automotive component of claim 14, wherein the automotive component is one of a differential component and a drive axle component.
 16. A nodular iron alloy consisting essentially of: about 3.1 to about 3.3 weight percent carbon; about 2.7 to about 4.3 weight percent silicon; about 0.15 to about 0.40 weight percent manganese; 0 to about 0.10 weight percent magnesium; 0 to about 0.2 weight percent nickel; 0 to about 0.4 weight percent copper; 0 to about 0.30 weight percent chromium; 0 to about 0.03 weight percent phosphorus; 0 to about 0.02 weight percent sulfur; and the balance iron.
 17. The nodular iron alloy of claim 16, wherein the silicon is provided in an amount of about 3.5 to about 4.1 weight percent.
 18. The nodular iron alloy of claim 17, wherein the nodular iron alloy has an ultimate tensile strength greater than 620 MPa as-cast.
 19. The nodular iron alloy of claim 18, the nodular iron alloy being substantially free of cobalt and molybdenum, wherein the iron is present in an amount of 65-85% of a ferrite microstructure and in an amount of 15-35% of a pearlite microstructure, and wherein the iron surrounds a plurality of graphite nodules.
 20. An automotive component created from a nodular iron alloy according to claim
 16. 